The present invention relates to an exhaust gas purifying apparatus for an internal-combustion engine, and more specifically to an exhaust gas purifying apparatus for an internal-combustion engine which can precisely determine deterioration of a nitrogen oxide (NOx) catalyst.
Conventionally, a NOx purifying device including a NOx trapping agent that is located in the exhaust system of the internal-combustion engine decreases NOx in the exhaust gas. Japanese Patent Application Unexamined Publication (Kokai) No. H10-299460 discloses a technique for determining deterioration of the NOx purifying device based on a delay time of outputs of the oxygen density sensors disposed upstream and downstream of the NOx purifying device when the fuel control air-fuel ratio of the internal-combustion engine has been set richer than a stoichiometric air-fuel ratio after the lean-burn operation is performed for a given time period. More specifically, as for this conventional technique, deterioration of the NOx purifying device is determined based on a fact that the delay time from the time the output of the upstream oxygen density sensor has changed to rich to the time the output of the downstream oxygen density sensor changes to rich is relatively large when the NOx trapping capability of the NOx purifying device is high, whereas such delay time becomes smaller as the NOx absorption capability degrades.
However, when the density of the sulfur oxide (SOx) becomes high in the peripheries of the oxygen density sensor, the output characteristic of the oxygen density sensor may change. Accordingly, from time to time, the conventional technique cannot precisely determine deterioration of the NOx purifying device when the engine is driven with fuel that contains high-density-sulfur. In other words, when the fuel control air-fuel ratio of the internal-combustion engine has been set richer after the lean-burn operation for a given time period, the output delay time of the oxygen density sensor located downstream of the NOx purifying device may become smaller due to the influence of SOx even if trapping capability of the NOx purifying device is sufficiently high. As a result, the NOx trapping capability cannot be correctly measured.
Thus, there is a need for a system that can correctly measure the NOx trapping capability of the NOx purifying device even if the engine is driven with fuel that contains high-density-sulfur.
According to an aspect of the present invention, an exhaust gas purifying apparatus for an internal-combustion engine is provided in order to achieve the aforementioned objective of the invention. The exhaust gas purifying apparatus comprises, in an exhaust system of the internal-combustion engine, an exhaust gas purifier or exhaust gas cleaner that includes a nitrogen oxide purifier for purifying nitrogen oxide contained in exhaust gas during a lean operation in which an air-fuel ratio of air-fuel mixture to be supplied to the internal-combustion engine is set leaner. The apparatus also includes an oxygen density sensor which is disposed downstream of the exhaust gas purifier. The exhaust gas purifying apparatus comprises a sulfur oxide detector for detecting density of sulfur oxide in the exhaust gas purifier, sulfur-poisoning recovery means for recovering the exhaust gas purifier from sulfur poisoning when the sulfur oxide detector determines that the sulfur oxide density is high. The exhaust gas purifying apparatus further comprises a lean operation limiter for limiting the lean operation of the internal-combustion engine to such operation region that is not significantly influenced by the sulfur poisoning after the exhaust gas purifier is recovered from the sulfur poisoning by the sulfur-poisoning recovery means. The exhaust gas purifying apparatus further comprises deterioration determining means for determining deterioration of the exhaust gas purifier based on output of the oxygen density sensor during the limited lean operation of the internal-combustion engine.
According to the invention, the operation of the internal-combustion engine is limited to an operation region having less sulfur exhaust by the lean operation limiter after the exhaust gas purifier is recovered from the sulfur poisoning. Accordingly, the deterioration determination upon the exhaust gas purifier could be performed in such condition that is not significantly influenced by the sulfur poisoning. Thus, it is possible to precisely measure the nitrogen oxide trapping capability of the exhaust gas purifier during the operation even with the high-density-sulfur containing fuel. Specifically, the operation is limited to such region in which the engine load is low, the engine rotational speed is low, the gear is in a high position and the vehicle speed is intermediate. The operation region is limited to prohibit the lean operation when the temperature of the internal-combustion engine is low leading to a low temperature of the exhaust gas. Trapping of the sulfur oxide (SOx) to the exhaust gas purifier (called xe2x80x9csulfur poisoningxe2x80x9d) relates with the exhaust gas temperature.
According to another aspect of the invention, the exhaust purifier in the exhaust gas purifying apparatus for the internal-combustion engine comprises a three-way catalyst and the nitrogen oxide purifier and comprises a second oxygen density sensor located between the three-way catalyst and the nitrogen oxide purifier. The sulfur oxide detector detects the sulfur oxide density through the second oxygen density sensor.
According to further aspect of the invention, the exhaust gas purifying apparatus for the internal-combustion engine comprises limitation canceling means for canceling the limitation by the lean operation limiter when the deterioration determining means determines that the exhaust gas purifier is normal. Thus, it is possible to return to a usual operation when it is determined that the exhaust gas purifier is normal.